Re: [PATCH 2/4] tcp: Re-introduce inactivity timeouts based on a clock algorithm

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Feb 05, 2026 at 01:17:28AM +0100, Stefano Brivio wrote:
> Nits only:
> 
> On Wed,  4 Feb 2026 21:41:35 +1000
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > We previously had a mechanism to remove TCP connections which were
> > inactive for 2 hours.  That was broken for a long time, due to poor
> > interactions with the timerfd handling, so we removed it.
> > 
> > Adding this long scale timer onto the timerfd handling, which mostly
> > handles much shorter timeouts is tricky to reason about.  However, for the
> > inactivity timeouts, we don't require precision.  Instead, we can use
> > a 1-bit page replacement / "clock" algorithm.  Every INACTIVITY_INTERVAL
> > (2 hours), a global timer marks every TCP connection as tentatively
> > inactive.  That flag is cleared if we get any events, either tap side or
> > socket side.
> > 
> > If the inactive flag is still set when the next INACTIVITY_INTERVAL expires
> > then the connection has been inactive for an extended period and we reset
> > and close it.  In practice this means that connections will be removed
> > after 2-4 hours of inactivity.
> > 
> > This is not a true fix for bug 179, but it does mitigate the damage, by
> > limiting the time that inactive connections will remain around,
> > 
> > Link: https://bugs.passt.top/show_bug.cgi?id=179
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > ---
> >  tcp.c      | 52 ++++++++++++++++++++++++++++++++++++++++++++++++----
> >  tcp.h      |  4 +++-
> >  tcp_conn.h |  3 +++
> >  3 files changed, 54 insertions(+), 5 deletions(-)
> > 
> > diff --git a/tcp.c b/tcp.c
> > index f8663369..acdac7df 100644
> > --- a/tcp.c
> > +++ b/tcp.c
> > @@ -198,6 +198,13 @@
> >   *   TCP_INFO, with a representable range from RTT_STORE_MIN (100 us) to
> >   *   RTT_STORE_MAX (3276.8 ms). The timeout value is clamped accordingly.
> >   *
> > + * We also use a global interval timer for an activity timeout which doesn't
> > + * require precision:
> > + *
> > + * - INACTIVITY_INTERVAL: if a connection has had no activity for an entire
> > + *   interval, close and reset it.  This means that idle connections (without
> > + *   keepalives) will be removed between INACTIVITY_INTERVAL s and
> 
> Probably easier to read: ... seconds
> 
> > + *   2*INACTIVITY_INTERVAL s after the last activity.
> 
> same here.

Good idea, done.

> 
> >   *
> >   * Summary of data flows (with ESTABLISHED event)
> >   * ----------------------------------------------
> > @@ -333,7 +340,8 @@ enum {
> >  
> >  #define RTO_INIT			1		/* s, RFC 6298 */
> >  #define RTO_INIT_AFTER_SYN_RETRIES	3		/* s, RFC 6298 */
> > -#define ACT_TIMEOUT			7200
> > +
> > +#define INACTIVITY_INTERVAL		7200		/* s */
> >  
> >  #define LOW_RTT_TABLE_SIZE		8
> >  #define LOW_RTT_THRESHOLD		10 /* us */
> > @@ -2254,6 +2262,8 @@ int tcp_tap_handler(const struct ctx *c, uint8_t pif, sa_family_t af,
> >  		return 1;
> >  	}
> >  
> > +	conn->inactive = false;
> > +
> >  	if (th->ack && !(conn->events & ESTABLISHED))
> >  		tcp_update_seqack_from_tap(c, conn, ntohl(th->ack_seq));
> >  
> > @@ -2622,6 +2632,8 @@ void tcp_sock_handler(const struct ctx *c, union epoll_ref ref,
> >  		return;
> >  	}
> >  
> > +	conn->inactive = false;
> > +
> >  	if ((conn->events & TAP_FIN_ACKED) && (events & EPOLLHUP)) {
> >  		conn_event(c, conn, CLOSED);
> >  		return;
> > @@ -2872,18 +2884,50 @@ int tcp_init(struct ctx *c)
> >  	return 0;
> >  }
> >  
> > +/**
> > + * tcp_inactivity() - Scan for and close long-inactive connections
> > + * @:	Execution context
> 
>  * @c:		Execution context
>  * @now:	Current timestamp

Oops, fixed.

> > + */
> > +static void tcp_inactivity(struct ctx *c, const struct timespec *now)
> > +{
> > +	union flow *flow;
> > +
> > +	if (now->tv_sec - c->tcp.inactivity_run < INACTIVITY_INTERVAL)
> > +		return;
> > +
> > +	debug("TCP inactivity scan");
> > +	c->tcp.inactivity_run = now->tv_sec;
> > +
> > +	flow_foreach(flow) {
> > +		struct tcp_tap_conn *conn = &flow->tcp;
> > +
> > +		if (flow->f.type != FLOW_TCP)
> > +			continue;
> > +
> > +		if (conn->inactive) {
> > +			/* No activity in this interval, reset */
> > +			flow_dbg(conn, "Inactive for at least %us, resetting",
> > +				 INACTIVITY_INTERVAL);
> > +			tcp_rst(c, conn);
> > +		}
> > +
> > +		/* Ready to check fot next interval */
> 
> for

Wow, I really wasn't typing good that day.  Fixed.

> 
> > +		conn->inactive = true;
> > +	}
> > +}
> > +
> >  /**
> >   * tcp_timer() - Periodic tasks: port detection, closed connections, pool refill
> >   * @c:		Execution context
> >   * @now:	Current timestamp
> >   */
> > -void tcp_timer(const struct ctx *c, const struct timespec *now)
> > +void tcp_timer(struct ctx *c, const struct timespec *now)
> >  {
> > -	(void)now;
> > -
> >  	tcp_sock_refill_init(c);
> >  	if (c->mode == MODE_PASTA)
> >  		tcp_splice_refill(c);
> > +
> > +	tcp_inactivity(c, now);
> >  }
> >  
> >  /**
> > diff --git a/tcp.h b/tcp.h
> > index 24b90870..e104d453 100644
> > --- a/tcp.h
> > +++ b/tcp.h
> > @@ -21,7 +21,7 @@ int tcp_tap_handler(const struct ctx *c, uint8_t pif, sa_family_t af,
> >  int tcp_listen(const struct ctx *c, uint8_t pif, unsigned rule,
> >  	       const union inany_addr *addr, const char *ifname, in_port_t port);
> >  int tcp_init(struct ctx *c);
> > -void tcp_timer(const struct ctx *c, const struct timespec *now);
> > +void tcp_timer(struct ctx *c, const struct timespec *now);
> >  void tcp_defer_handler(struct ctx *c);
> >  
> >  void tcp_update_l2_buf(const unsigned char *eth_d);
> > @@ -38,6 +38,7 @@ extern bool peek_offset_cap;
> >   * @rto_max:		Maximum retry timeout (in s)
> >   * @syn_retries:	SYN retries using exponential backoff timeout
> >   * @syn_linear_timeouts: SYN retries before using exponential backoff timeout
> > + * @inactivity_run:	Time we last scanned for inactive connections
> >   */
> >  struct tcp_ctx {
> >  	struct fwd_ports fwd_in;
> > @@ -47,6 +48,7 @@ struct tcp_ctx {
> >  	int rto_max;
> >  	uint8_t syn_retries;
> >  	uint8_t syn_linear_timeouts;
> > +	time_t inactivity_run;
> >  };
> >  
> >  #endif /* TCP_H */
> > diff --git a/tcp_conn.h b/tcp_conn.h
> > index 21cea109..7197ff63 100644
> > --- a/tcp_conn.h
> > +++ b/tcp_conn.h
> > @@ -16,6 +16,7 @@
> >   * @ws_from_tap:	Window scaling factor advertised from tap/guest
> >   * @ws_to_tap:		Window scaling factor advertised to tap/guest
> >   * @tap_mss:		MSS advertised by tap/guest, rounded to 2 ^ TCP_MSS_BITS
> > + * @inactive:		No activity within the current INACTIVITY_INTERVAL
> >   * @sock:		Socket descriptor number
> >   * @events:		Connection events, implying connection states
> >   * @timer:		timerfd descriptor for timeout events
> > @@ -57,6 +58,8 @@ struct tcp_tap_conn {
> >  	(conn->rtt_exp = MIN(RTT_EXP_MAX, ilog2(MAX(1, rtt / RTT_STORE_MIN))))
> >  #define RTT_GET(conn)			(RTT_STORE_MIN << conn->rtt_exp)
> >  
> > +	bool		inactive	:1;
> > +
> >  	int		sock		:FD_REF_BITS;
> >  
> >  	uint8_t		events;
> 
> -- 
> Stefano
> 

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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